Why would a girl keep running back to a toxic relationship?

She's been with the same cheating and abusive guy for 4 years. No matter how many times they break up (possibly over a hundred by now), she always run back to him. No amount of convincing from her friends will make her leave him for good. Even though she KNOWS he's not good for her. He's the only guy she's been dating since 2017.
Why would a girl keep running back to a toxic relationship?
Post Opinion